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Some patterns of shape change controlled by eigenstrain architectures

Abstract : Nature provides an unlimited source of inspiration for engineers, either by exhibiting new solutions to existing problems or by challenging them to develop systems displaying new functionalities. Recent advances in the characterization and modeling of natural systems reveal new design principles, which can be increasingly mimicked by engineers thanks to the progress in the production and modeling of man-made materials. In this thesis, we are inspired by biological actuators (for example the pine cone) which change their shape under an external fluctuating stimulus as a result of their material architecture. Our goal is to explore the design space of the morphing of solid objects controlled by an imposed distribution of inelastic strain (eigenstrain). We focus on elongated and thin objects where one dimension is either much bigger or much smaller than the other two (rods and sheets) and restrict ourselves to the framework of linear elasticity. Patterns of shape change are usually induced by large transformations, which requires considering a nonlinear dependency between strain and displacements. This requires the use of numerical methods in order to predict the morphing patterns. We looked at relaxation of springs, energy minimization and finite-elements. These patterns were also illustrated using experimental methods such as pre-straining, thermal expansion and swelling. In the context of rod-like objects, two fundamental morphers are studied displaying bending and twisting respectively: benders and twisters. The standard mirror-symmetric bilayer eigenstrain architecture of benders can be smoothened in order to lower interfacial stress and modified in order to produce longitudinally graded or helical benders. By stacking benders in a honey-comb like manner, the relatively small mid-deflection of benders is geometrically amplified and produces relatively large displacements. According to finite-element simulations, the proposed rotationally-symmetric eigenstrain architecture of twisters displays a stretching-twisting instability, which is analyzed using energetical arguments. Similarly to benders, twisting can be varied along the longitudinal direction by grading the material properties along the twister. By combining benders and twisters, an arbitrary configuration of a rod can be obtained. In the context of sheets, we focus on diffusion-driven morphing, where the eigenstrain is applied progressively instead of instantaneously as motivated by experiments on thermo-responsive polymer bilayers. This leads to long-side rolling of rectangular shapes (instead of the standard short-side rolling of benders) and reveals a complex multi-step morphing process in the case of star shapes, where the edges wrinkle and bend and the initially flat star eventually folds into a three-dimensional structure (for example a pyramid). With the progress in designing new materials, the morphers presented in this thesis could be used in different fields, including the design of macroscopic structures for Architecture.
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Submitted on : Monday, September 28, 2015 - 1:07:06 PM
Last modification on : Thursday, November 19, 2020 - 3:54:18 PM
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Sébastien Turcaud. Some patterns of shape change controlled by eigenstrain architectures. Materials. Université Grenoble Alpes, 2015. English. ⟨NNT : 2015GREAI005⟩. ⟨tel-01206053⟩

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